This invention relates to hydroforming of dual wall tubular products such as engine exhaust conduits, and particularly to hydroforming such products to obtain a uniform controlled spacing or gap between the inner and outer walls.
In U.S. Pat. No. 5,170,557 is set forth a hydroforming process for forming a dual wall conduit, with a minimum air gap between the inner and outer walls being assured. Often it is desirable to have a predetermined uniform air gap, not just a minimum gap, between the walls. Specifically, for example, an auto manufacturer may want a dual wall engine exhaust conduit with a two and one-half inch outer wall diameter and a two and one-quarter inch inner wall diameter, and a uniform spacing or gap between them. The prior known technology does not enable that to be assured. This is particularly so when the dual wall conduit is bent into various nonlinear configurations, as is usually done. Such bending tends to cause reduction in the conduit diameter, i.e., necking down of the walls, at the bend zones, and formation of wrinkles or bulges adjacent the bend zones. When subsequently hydroform expanded in the conventional way in which only the outer wall is expanded outwardly to the surface of the hydroforming die cavity, the result is an air gap of differing amounts and configurations along the conduit length.